This disclosure relates to electrochemical cells, and, more particularly, to an electrochemical hydrogen compressor for recirculating an amount of hydrogen gas above the stoichiometric feed ratio through a fuel cell.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. Proton exchange membrane electrolysis cells can function as fuel cells by electrochemically reacting hydrogen with oxygen to generate electricity and as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, a section of a proton exchange membrane fuel cell is shown generally at 10 and is hereinafter referred to as “cell 10.” In cell 10, hydrogen gas 12 and reactant water 14 are introduced to a hydrogen electrode (anode) 16, while oxygen gas 18 is introduced to an oxygen electrode (cathode) 20. Hydrogen gas 12 for fuel cell operation can originate from a hydrocarbon, natural gas, or any other hydrogen source. Hydrogen gas 12 electrochemically reacts at anode 16 to produce hydrogen ions (protons) and electrons such that the electrons flow from anode 16 through an electrically connected external load 24 and such that the protons migrate through a membrane 22 to cathode 20. At cathode 20, the protons and electrons react with the oxygen gas to form water, which additionally includes any reactant water 14 that migrates through membrane 22 to cathode 20. The electrical potential across anode 16 and cathode 20 can be exploited to power external load 24.
A similar configuration as is depicted in FIG. 1 for a fuel cell is often used for electrolysis cells. In an anode feed water electrolysis cell (not shown), reactant water is fed to a cell at an oxygen electrode (anode) to form oxygen gas, electrons, and hydrogen ions (protons). The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (cathode). The oxygen gas and a first portion of the water are discharged from the cell, while protons and a second portion of the water migrate across a proton exchange membrane to the cathode where hydrogen gas is formed. In a cathode feed electrolysis cell (not shown), water is fed at the hydrogen electrode, and a portion of the water migrates from the cathode across the membrane to the anode where protons and oxygen gas are formed. A portion of the water is discharged from the cell at the cathode side without passing through the membrane. The protons migrate across the membrane to the cathode where hydrogen gas is formed.
An electrochemical cell system (either a fuel cell system or an electrolysis cell system) includes one or more individual cells arranged in a stack with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged in ionic communication, each including a cathode, a proton exchange membrane, and an anode. In certain arrangements, the anode, cathode, or both are gas diffusion electrodes that facilitate gas diffusion to the membrane. Each cathode/membrane/anode assembly (hereinafter “membrane electrode assembly,” or “MEA”) may be supported on one or both sides by flow fields that may comprise screen packs and/or bipolar plates. Such flow fields facilitate fluid movement and membrane hydration and provide mechanical support for the MEA. Because a differential pressure often exists in the cells, compression pads or other compression means are often employed to maintain uniform compression in the cell active area, i.e., the electrodes, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.
While existing electrochemical cell systems are suitable for their intended purposes, there still remains a need for improvements. Some of the improvements needed include the use of devices that provide for the recirculation of hydrogen gas through a fuel cell in order to stabilize or eliminate the decay of the fuel cell while limiting the complexity of the system and maintaining the system reliability.